Power factor is defined as the cosine of the phase angle between an instantaneous alternating current (ac) voltage and current, considering only fundamental values of the ac voltages and currents. Unity power factor is achieved when the phase angle between the voltage and current is zero. It is desired to have a unity power factor when drawing power from a utility, so that only real power is drawn, and no reactive power. A utility's investment for generation and distribution equipment will be minimized only if real power is drawn by users, which amounts to the user's power factor being unity.
Converters in the form of rectifiers distort ac current drawn from the utility supply, leading to non-sinusoidal current waveforms that introduce harmonics, other than the fundamental, that are undesirable to the operation of the utility, as well as contribute to additional losses that do not exist when only sinusoidal currents are drawn from the utility. Therefore, power factor correction is of importance for motor-drive applications in many countries because of regulations and incentives for manufacturers of electronics to build into their systems a capability for unity power factor operation and sinusoidal current draw.
Common ways to incorporate unity power factor operation and sinusoidal current draw are: (1) to provide a separate unity power factor correction (UPF) circuit, which is an expensive approach and takes additional space and volume for installation, and (2) operating a full-bridge controlled rectifier in boost mode, which is also an expensive solution. A solution was patented by Krishnan Ramu (U.S. Pat. No. 7,271,564, issued Sep. 18, 2007) for addressing these challenges; this solution employed a single transistor for controlling a two-phase machine. A disadvantage of the patented solution pertains to the limited torque-generating region of a two-phase machine. More specifically, when phase A is conducting, phase B must conduct also. The torque productions of these two phases are of opposite polarity, some of the time. Therefore, the torque production in a two-phase machine has a reduced output.
Although the currents in each phase of a two-phase machine may be made unequal, so the torque contributions from the two phases are unequal in magnitude, the net torque is also reduced. Moreover, the reduced torque is produced in every alternate torque-generation region of either phase A or B, whichever can produce the maximum torque compared to the other phase.
Assume the phase-B winding of a two-phase machine has less turns than the phase-A winding. The unequal number of turns between the phases makes phase B the auxiliary phase, with smaller torque generating capability compared to phase A. The net torque produced by the two is less in the machine with unequal number of turns compared to a machine with equal number of turns in the phase windings.